This is just to get this thread started and will be posted in sections due to the length of the tutorial. Please feel free to add to this thread the expand the knowledge base for everyone to benefit. I know some of you will have more info on the side drafts and some will have more info on the downdrafts.

Okay....This is going to be the Short Course in what you need to know to Tune and Jet your Weber Carbs successfully...This also applies to Mikuni, Solex, Etc....This is not specific info about specific cars or giving you magic jetting numbers. This is the process we use to tune a car when it is in our shop. I have over 25 years experience tuning these carbs and there is no magic to it. It is just a step by step process that once you get the hang of it you will be able to make your carbs work right every time. It involves more than just Jetting the carbs and in fact that step is the easiest....Following these steps and rules will also make it much easier when you are purchasing jets for your carb...This applies to a single 2 Barrel Downdraft 32/36 Carb or a set of Triple DCOE carbs or even a set of 4 Downdrafts on a V8 engine....The 2 books listed at the bottom of this page are invaluable tools for learning all the basics of your carbs and the various metered tuning pieces that are used in them. It is well worth the small investement in money and time to buy and read these books to help you better understand the carbs you spent all that money on.

I am not going to explain everything about these carbs. The Weber Factory Tuning manual has all the good technical information already written. This is the backyard mechanics guide to the basics you need to know to make your carbs run right....

Step-1: If you are having a problem with how the engine runs...Poor idle quality, stalling, etc then this is the first thing to do. CHECK FOR VACUUM LEAKS ! This is particulary true of new installations where you just purchased and installed a conversion kit and you are having problems. You have to remember that most Weber conversion have been around for many years and have been installed on hundreds of cars and trucks. They work. The likelyhood of having a defective carb out of the box is about like being hit by lightning. If you are having a problem on a new install it is most likely this problem or one of the steps below... See the Vacuum leak tech page for this info. DO NOT OVERLOOK this as a problem...This is the #1 problem with any Weber carb conversion. * Do a compression test..If the compression is off by more than 10% per hole you will have a really bad time tuning the carbs.* Always start with a fresh set of Spark Plugs. So not clean them...Start fresh. This will give you a good reading of fuel mixture and enable you to make a good judgement of what you need to do. * Set the Timing...Generally speaking you need a little more advance with a Weber than with the stock carb. A good place to start for most cars with Webers on pump gas is 12-14 Degrees advance at 1000 RPM idle and 36 Degrees total advance by 3000 RPM...This is not cast in stone and you need to make sure your car does not Detonate at these setting. If you get any detonation then you need to back off the timing...

Step-2: If you have a single carb you can skip this step...If you have multiple carbs then the 1st thing you need to do is make sure the carbs are Synchronized properly using a Correct Synch tool. This is CRITICAL to multiple carb performance...I don't care how good you think you are at synching a set of multiple carbs by ear (I'm very good and I'm not half as good as the tool is.) you are not close enough. The Synch tool will also alert you to other problems in the system like Bent throttle shafts, etc.... Linkage...This cannot be stressed too highly. Badly designed or Worn linkage is the #1 cause of problems with Multiple carb systems. More people have given up on Dual and Triple carb systems because of linkage than any other problems combined. If the carbs are not hooked together in a fashion that has no slop and allows minute adjustment between the carbs you are always going to have problems. Poor idle quality and "sticky high idle" problems are the most common symptom of this. A slight imbalance between the carbs at 1/3 throttle and up is not really noticeable to most people but a miniscule difference at idle will cause all sorts of problems. So in short...Get the carbs synched right FIRST. Repair or replace the linkage as needed to make them work together properly.

Step-3: Know what is in your carb NOW. Do not rely on a list on a website or a book that says what your carb might have come with. Actually remove the jets and chokes and check the sizes and WRITE THEM DOWN. Use this form to write it in so you know what you have. You cannot make jet decisions unless you kow what you are starting with...

Step-4: Make Sure your Ignition system is working properly. Remember...A Weber carb is always a performance upgrade and is designed to deliver more fuel and air and make more HP then the stock carb did. This means you need to have adequate spark to burn the extra fuel or you will have problems tuning and jetting your carbs. This is especially true of old British cars and cars that have point type igntion systems. A lack of spark or weak spark will make it impossible to tune the carbs properly or to their maximum potential. This is especially true of all multiple carb installations. You cannot hang multiple carbs on any engine without an increase in spark output and even hope to burn the extra fuel being delivered. A good electronic Ignition system to replace points and the addition of an MSD unit and good coil and wires are the best way to go. See our Ignition tech page for more details.

Step-5: Make sure that your Fuel Delivery is right....Once again, you cannot tune carbs that do not have adequate or proper fuel delivery. See our Fuel pump Tech Page for this information. Do not ignore this step.

Step-6: Okay...Now that you have everything else sorted out ( Right ? You did do all that other boring stuff didn't you ? ) you can actually start tuning the carbs...The good news is that you probably made the car run good enough by doing those other steps that there is not alot else to do..... If your car is still not running properly or you feel that you should have more power (Be realistic about that. A 4 cylinder 2.0 with a 32/36 is only going to make about 120-125 HP max so don't expect miracles.) then it is time to jet and tune the carb(s) You need to establish if you are running lean or rich. There is no way around this. There are a few easy ways to establish this and it is important to know otherwise you cannot get different jets to cure the problem. You can be running Rich at idle and Lean on the main circuit and Vice Versa...You need to do a plug check....Unless you have access to a 3 Gas or 4 Gas anaylzer or Smog Machine to do your tuning then reading the plugs is the next best thing..Another good option is to install a Halmeter AF30 Air Fuel Ratio Gauge to help you with tuning. It is fast and accurate and will take alot of the guesswork out of it. This is a particularly valuable tool for cars that get track use so that you make sure you do not lean out and put a hole in a piston...

You need to check your spark plugs to see if you running rich or lean. White plugs are lean and Black plugs are Rich. Ideally you want the ceramic part of the plug a nice Dark Tan to medium Brown color with a slightly darker ring right at the base of the threads of the plug. A new set of plugs may take a few minutes to get some color on it...

Idle Mixture and Fuel Mixture:There are alot of theories about setting idle mixture adjustment on a Weber carb to determine Rich or Lean jetting....A Weber will run correctly with the mixture screws from 1/2 turn to 3 turns out...The late style DCOE carbs with the extra Air Bleed Screws on the top take 2.5 to 3 turns to operate correctly...So you cannot say that because it is 1/2 turn out that you need smaller jets....You need to find what is right for that particular engine...An engine with a strong vacuum signal will draw more fuel in with less turns of the screw than an engine with a weak vacuum signal will...This is the kind of thing that throws the "certain number of turns" theory out the window..

The Mixture screw and idle circuit is CRITICAL to the overall driveability of the car. It does not just control idle but the entire low speed running and part throttle transition. The mixture screw lets in an ALREADY MIXED volume of Fuel and Air to the engine. This is not an Air Screw. The more you open it the more mixed fuel and air enters the engine. Clockwise is Leaner and Counter-Clockwise is Richer.

The mixture screew is very easy to set whether it be a downdraft, sidedraft or multiple sidedrafts. Start with the screw or screws out 1.5 turns....Start the car and let it warm up. Set the Idle SPEED to approx 900-1000 RPM..Make sure multiple carbs are synchronized...Turn the mixture screws in until the idle starts to stumble and get rough...On a sidedraft with 2 screws do them each a little at a time....Then back them out until the best idle quality is acheived. This is a very simple operation...The car should idle well and small adjustments leaner (Turning them in) should make the idle drop off. Opening the screws more should make it a bit richer but it should still idle. Assuming you have no vacuum leaks this is a very simple process. If you cannot get a good adjustment on these screws and you have to open the idle speed screws quite a bit to get the engine to idle there is a good chance you have a Vacuum Leak and you need to fix it.

Specific Information on Mixture Screw Setting for:32/36 DGV, DGEV Carbs. If you have to open the mixture screw more than 2 turns on a 32/36 DGV or 38 DGES your idle jets are too small...If you have to shut them below 1/2 turn they are too big....

38 DGES. If you have to open the mixture screw more than 2 turns on a 32/36 DGV or 38 DGES your idle jets are too small...If you have to shut them below 1/2 turn they are too big....Also the 38 DGES can be a little tricky because you are idling on both barrels at the same time. You have 2 mixture screws and they will not be set the same on most cars. This is because the plenum type manifold that they are on distributes fuel unevenly. By having 2 mixture screws you are delivering fuel from 2 places in the intake manifold. The Mixture screw closest to the engine will no doubt need to be in much farther than the outer mixture screw. This is Okay. I do not recommend Stagger jetting the idle circuit on a 38 DGES (In other words, do not use 2 different size idle jets even though you need to adjust the screws differently. This can cause part throttle dirveability issues.)

DCOE Series and IDF / IDA. On a DCOE or multiple DCOE's you should be between 3/4 turn and 1.5 turns out for all older model DCOE's (DCOE 2, 9, 18, etc.) and 2 1/4 to 3 turns out for late style DCOES (151 and 152 with air bleed screws under the white caps.)

Idle Jets and tuning the idle circuit: Very Simply..The bigger the number the richer the jet. A 50 Idle jet is a .5mm fuel hole. On the DCOE and IDA series carbs they have 2 numbers on them like 50F8. The 50 refers to the .5mm fuel hole and the F8 Refers to the Air Bleed hole in the side. A DGV, DFV, DGES mix the air internally in the carb and do not have these air bleed holes in the idle jet. In a DCOE you are acutally tuning the Air and Fuel for the idle circuit with the idle jets. This is a little more tricky but not too bad...Basically you can tune just about any car in the world with an F8 or an F9 idle jet. ( YES there are exceptions to this and I am well aware of them so don't send me nasty e-mails saying that your Mini only runs on F6 idles. This is a basic tuning primer and there is no need to get into vehicle specific problems.) The F8 is Leaner (Has a larger Air hole) than the F9...Soooooo. You can have a 50F8 and a 50F9 and both have the same amount of fuel but the 50F9 has a richer MIXTURE (Less Air to the same volume of fuel.) Generally speaking you should start with an F8 and play with the fuel size until you get close then experiment with an F9 to see if that works any better...

IDF carbs are similar to the DGV and DFV series in that the air is mixed internally. There is no F Number to deal with. The IDA carbs have no Air Bleed Hole but do have an F number to identify this feature. It is an F10. The Air Bleed for the IDA carb is in the Idle Jet holder and it is metered similar to the DCOE. For those of you with multiple IDA or Rotary engines running a single 48 IDA you usually know what you are doing so I won't go into all the details on this.,..

Idle jets are in .5 steps...50,55,60,65 etc. The bigger the Idle jet number the richer the jet. 1 step in idle jet size can make a HUGE differerence. Do not go up or down more than 1 step at a time when tuning the idle circuit.

Choke Tubes and the Main Circuit:Okay...So now your car should idle correctly, the carbs are synched, the timing is set, everything is good....Time to get the main circuit right. Drive the car. It should come off of idle good and transition to the main circuit smoothly...If it falls on its face and will not take any throttle or runs better if you back off the then you could have a few different problems depending the type of carb. Before you blame the carb make sure you have the Fuel delivery right and the Spark Timing. If you do not have enough advance you will have this problem and it will have nothing to do with the carb tuning.

Thanks, AaronLife is about making dreams come true, stop dreaming and make life happen. MY_YOUTUBE_VIDEOSTo keep an opened mind- speculation is devine. Assumptions always lead to a dead end!

32/36 DGV, DGEV, DFV and DFEVThese carbs have fixed venturis so the choke size is not an issue. The pump jets are also not a problem. Do not mess with them. If you have a flat spot or hesitation when you first take off then it is likely that the idle circuit is too lean. If you know you have the idle circuit right then the primary main jet is too small. Increase the main jet size 2 steps at a time. (150 main is a 1.5mm hole...Main jets are in steps of 5. eg: 150, 155,160,etc.) Keep checking the plugs after driving it for a few minutes (Do not let it idle when checking the spark plug color for the main jets. Drive the car above 2000-3000 RPM for a few minutes then shut the engine off before letting it idle then check the plugs. If the car then transitions fine but hesitates or falls on it face when the secondary is opened you need to work on the Secondary main jet....A good option to playing around with your 32/36 Weber jetting is to just get one of our Custom Performance Jetting Kits....We offer these for all Single 32/36 DGV and DGEV applications operating from Sea Level to approx 4000 Ft...Above 4000 Ft you start having more altitude problems and the vehicle needs to be tuned as outlined here. Another good option is to install a Halmeter AF30 Air Fuel Ratio Gauge to help you with tuning. It is fast and accurate and will take alot of the guesswork out of it. This is a particularly valuable tool for cars that get track use so that you make sure you do not lean out and put a hole in a piston...

38/38 DGES These carbs have fixed venturis so the choke size is not an issue. The pump jets are also not a problem. Do not mess with them. If you have a flat spot or hesitation when you firts take off then it is likely that the idle circuit is too lean. If you know you have the idle circuit right then the main jet is too small....Increase the main jet size 2 steps at a time. (150 main is a 1.5mm hole...Main jets are in steps of 5. eg: 150, 155,160,etc.) Keep checking the plugs after driving it for a few minutes (Do not let it idle when checking the spark plug color for the main jets. Another good option is to install a Halmeter AF30 Air Fuel Ratio Gauge to help you with tuning. It is fast and accurate and will take alot of the guesswork out of it. This is a particularly valuable tool for cars that get track use so that you make sure you do not lean out and put a hole in a piston...

DCOE, IDF or IDAHere we go....This is the real tricky part. I feel that Sidedraft DCOE , IDF and IDA carbs are easy to tune but you have to have a real feel for Rich and Lean and understand how the carb works. To get a good understanding of this I cannot recommend too highly to get the Weber Factory Tuning manual and really understand what each component does. That said this is the real Cliff Notes version of tuning the power circuit of these carbs.

Choke Tube or Main Venturi size is the basis for everything in tuning DCOE, IDA and IDF carbs. If you get the Venturis wrong you will never get it running right. Too big and you will always have a flat spot that you cannot tune...Too small and it will always run rich and not make any power. If you have poor throttle response at low RPM

This is a guideline only to get you started or to make sure you are not totally out of range making it very difficult to tune. There are many of you running cars successfully with choke sizes outside this range so don't e-mail me about it. The difficulty in tuning these types of cars is that there are many combinations that work well depending on the engine and the state of tune. That is why everything is tunable. You can tailor the carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc.....

Here is a very basic chart of what venturi sizes you need to popular vehicles and engine sizes for STREET USE...All out race engines are a totally different story. Use the engine size and HP rating to estimate what you need for your car or engine if it is not listed.

This is a guideline only to get you started or to make sure you are not totally out of range making it very difficult to tune. There are many of you running cars successfully with choke sizes outside this range so don't e-mail me about it. The difficulty in tuning these types of cars is that there are many combinations that work well depending on the engine and the state of tune. That is why everything is tunable. You can tailor the carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc.....

Engine / Car Approximate HP Carb or Carbs Starting Choke Tube Size

Datsun Z 2.8 - 3.0 250-280 Triple 45 DCOE 36mm

Datsun Z 2.4 - 2.8 200-250 Triple 40 DCOE 34mm

Do not try to choke down a DCOE carb that is too large to try and cure a problem...you will just create more problems....A 45 DCOE should NEVER need less than a 34mm choke to run properly. If it does then the carbs are too large or you have another problem...A 40 DCOE can use as small as a 28mm choke tube but chances are the performance will not be good with anything smaller than a 30mm choke....I have found that a correctly sized 40 DCOE application should always stat with a 30mm choke and do up to a 34mm max....Bigger or smaller than that and you probably have either the wrong size carbs or another problem that is causing you to tune outside of this range...This same info applies to IDF carbs as well...

Okay....This is going to be the Short Course in what you need to know to ...

3 pages later...:eek:

I'm just kiddin...I was thinking today about tearin into mine and redoin some jet sizing for Denver elevation. Plus...since my 32/36 sat in the forest , on a VW motor for about 2 years...maybe some inspection would be good. I just bolted it on 3 years ago and it's been purrin ever since.

hey i got a 32/36 thats draing fuel into the bottom end of my motor...any hints on where to start....thanks jOn

The bottom end is completely separate from the top end, separated by the head gasket...you may want to try a compression test. If you have a bad head gasket, That may be how fuel is getting into your bottom end.

Is there water in your oil or visa versa?

Start there and update us.

Thanks, AaronLife is about making dreams come true, stop dreaming and make life happen. MY_YOUTUBE_VIDEOSTo keep an opened mind- speculation is devine. Assumptions always lead to a dead end!

neither the oil is just starting to smell of fuel.. the last rig i had it on did the same thing the oil began to reak of fuel got thin and ruined a main bearing... I am trying to avoid this happening again... no water , no blown head gasket....just the oil looks thinner and smells of gas....even after a oil change it returns to this state.... on both trucks ..

I find this really hard to believe. The carb has a float bowl that contains the fuel, it has nowhere to go. Maybe you have a float problem in the carb that is overflowing it into the motor, but i find that hard to believe if this carb is on a running motor.

Ok i will simple it out for everyone....i pull the dipstick its runny and smells of fuel.....i can only assume i didn't poor fuel into the dip stick so i assume its coming from the carb....anyone had this issue, need help, it's washing my cylinders down....big time...bearing failure inevitable....it killed my 2.2 in my mazda.... is it too rich, I smell unburned fule when its running.....same as with my mazda....it got crappy mileage before....and kinda does on the datsun....any help before i ruin the quietest l16 i ever heard....or had....

Could the fuel pump diaphram be leaking? Wouldnt that let fuel into the crank case?

Good call.

If rings are good, then its more likely fuel pump. Where did this carb come from before the mazda, sounds way over jetted. Check your jet sizes, pretty simple to do. If you don't have the confidence to install the proper jets, get a new carb that IS properly jetted.

i think the jets too any idea where it should be close to i can change them.....alos the fuel pump is new and the rings are fine.....it came from weber redline its the second carb igot from them the first was so over jetted it wouldnt run on the mazda....

i've had a simillar problem but mine was in a 2.2 stroker 16v vw in my rabbit, i had an injector get stuck open so it would just dump fuel into the cyl' and the fuel would blow by the rings (that were brand new) and fill the crank case, and i have also had a simillar problem with an old 5.0L in one of my projects, the problem was the float getting stuck open. So i'd check and see if the carb is clean inside, floats not sticking, none of the vacuum ports internally are clogged backing the pressure inside up.

btw, im having a hell of a time trying to figure out EXACTLY where my electric choke should be wired, and how to adjust it? i have it wired to the factory electric choke wire from the factory harness, but it seems like it isnt doing a darn thing... i have to feather the throttle for about a minute atleast when i start it before i can even hold the throttle steady and have it stay running, and then it sits and sputters at about 2-3k rpm or it just WONT run until it hits running temps (takes about 4-5 min or so) and then i can let it idle and more it otherwise it just has absolutely no power at all and just dies if i let out the clutch. im about 5 seconds away from just throwing this pos weber in the trash and throwing my stock carb back on so it will run right, my only problem is with the hybrid swap i cant throw my stock fuel pump on the engine and im not sure how the stock carb will like the electric fuel pump and not sure exactly how to set up my fuel return?

(this is on a 20R hybrid engine in my 85 toyota, A 22r with 20r head on it...suppossed to have tons of power... i've yet to be impressed.. My 5.0L swap is calling my name louder and louder..i am seriously starting to lose faith in the 4cyl...)

Set to initial settings (mixture screw 1.25-1.5 turns from bottom, idle speed screw 1 turn in once contacting bellcrank), install. Start, warm-up engine, make sure choke is fully open, then adjust mixture screw to get fastest idle, bring idle speed back to spec with idle speed screw, tweak mixture screw to get fastest/smoothest idle, adjust as necessary with idle speed screw. Now check for off-idle lean-ness/richness and adjust accordingly. Not too hard, and no special tools are necessary.

With that said... I just snagged a brand new 38DGES for myself for christmas... and what I'm wondering in order to make things a little easier in the tuning process is if anyone has tuned one at altitude? I live in Colorado Springs which sits about 6,000 ft above sea level. I'm basically wondering what the best size jet will be, how much to advance the timing. Also some engine specific stuff, where did some of the mixture screws end up to run properly? I have a Z24 (yeah i know its a POS but it has big plans... don't worry.)

I didn't search for this one... but might as well ask while I'm in here... with the 38 DGES and the Z24, what would be the best fuel pressure to run at? This is for an autocross build so my guess is 3.5 psi, and I've seen some people that have done return line deletes in order to get the pressure set up right with an FPR.

I suck at searching... so feel free to flame on if i missed something somewhere... but at least link me to it plz!

With that said... I just snagged a brand new 38DGES for myself for christmas... and what I'm wondering in order to make things a little easier in the tuning process is if anyone has tuned one at altitude? I live in Colorado Springs which sits about 6,000 ft above sea level. I'm basically wondering what the best size jet will be, how much to advance the timing. Also some engine specific stuff, where did some of the mixture screws end up to run properly? I have a Z24 (yeah i know its a POS but it has big plans... don't worry.)

I didn't search for this one... but might as well ask while I'm in here... with the 38 DGES and the Z24, what would be the best fuel pressure to run at? This is for an autocross build so my guess is 3.5 psi, and I've seen some people that have done return line deletes in order to get the pressure set up right with an FPR.

I suck at searching... so feel free to flame on if i missed something somewhere... but at least link me to it plz!

Thanks in advance for any help guys!

Well, in my '83 720 KC 4x4 I had a Z24 with Z22 head and intake, Weber 38/38, Pacesetter header( only one I could find at that time), a small camshaft upgrade( the shop called it a 'RV' grind, about 10% more power was quoted) and a pair of MSD Blaster 2 coils.. The Weber came stock with 142's in the main's, 45's in the idles, and 185's in the air correctors.. which was a little lean for my engine. So after tuning, the carb contained 145's to 150's in the mains(depending on time of year, these Webers' are very picky about temp changes to stay in tune), the idles had to be changed to 55's-60's(to lean). Make sure that the jet's are installed in matching pairs since this carb is synchronous, (IE: 45-45, 55-55)Fuel pressure: 3.5 lbs is recomended maximum pressure in the Weber Bible. Advancement of timing is also recommended with a Weber installed.

As far as timing goes: each engine is unique. Start at the setting under the hood or in the service manual, advance the timing by 1-2 degrees at a time and then take on a road test. (I use a local street on a decent hill to test). Listen for a "ping" (kinda sounds like small pebbles or coins in a glass jar being shaken about). If there's not an audible 'Ping' noticed, go ahead and advance 1-2 more degrees and test again. Keep at this procedure until the 'ping' shows it self, then back the timing off by 1-2 degrees. That's the most advance to safely run. Someone else might be able to tell you a # for timing, but I just follow this per car. Hope this helps...

hey i got a 32/36 thats draing fuel into the bottom end of my motor...any hints on where to start....thanks jOn

When the Weber was installed, did you or whoever did the install remove the heat shield and phenolic spacer from under the origional carb??? If so, then it may be boiling the fuel out of the float bowl, through main jet openings and down the intake to the cylinder and past the rings. Happened to me before I knew about the heat shield thing....

Well, in my '83 720 KC 4x4 I had a Z24 with Z22 head and intake, Weber 38/38, Pacesetter header( only one I could find at that time), a small camshaft upgrade( the shop called it a 'RV' grind, about 10% more power was quoted) and a pair of MSD Blaster 2 coils.. The Weber came stock with 142's in the main's, 45's in the idles, and 185's in the air correctors.. which was a little lean for my engine. So after tuning, the carb contained 145's to 150's in the mains(depending on time of year, these Webers' are very picky about temp changes to stay in tune), the idles had to be changed to 55's-60's(to lean). Make sure that the jet's are installed in matching pairs since this carb is synchronous, (IE: 45-45, 55-55)Fuel pressure: 3.5 lbs is recomended maximum pressure in the Weber Bible. Advancement of timing is also recommended with a Weber installed.

As far as timing goes: each engine is unique. Start at the setting under the hood or in the service manual, advance the timing by 1-2 degrees at a time and then take on a road test. (I use a local street on a decent hill to test). Listen for a "ping" (kinda sounds like small pebbles or coins in a glass jar being shaken about). If there's not an audible 'Ping' noticed, go ahead and advance 1-2 more degrees and test again. Keep at this procedure until the 'ping' shows it self, then back the timing off by 1-2 degrees. That's the most advance to safely run. Someone else might be able to tell you a # for timing, but I just follow this per car. Hope this helps...